FIG. 16 is a perspective view of a conventional finger blood pressure meter. FIG. 17 is a cross-sectional view of a conventional blood pressure meter finger cuff. Several types of designs of finger cuffs are currently available.
The finger cuff shown in FIG. 17 has a wrapper constituting a sheet (or membrane) made of rubber or a soft plastic which is folded over on itself. Chamber 501, which contains compressed air, is formed by welding or bonding the ends of this sheet. Generally, the outside of chamber 501 has strips of cloth around its top and bottom ends which cause it to assume the shape of a cylinder when inflated. Cylindrical cuff or chamber 501, is enclosed within cylindrical case 500.
To measure a patient's blood pressure, the patient inserts his finger into the cylindrical cuff 501. When chamber 501 is filled with compressed air via the air tube connected to it, it expands like a balloon, adhering to and pressing against the finger. The blood pressure is measured by obstructing the flow of blood in the artery of the finger.
As seen in FIG. 16, the finger 400 inserted in the cylindrical cuff does not touch anything outside of the cuff nor does it rest on a fixed member or support. A patient often bends his finger, thus not relaxing the finger for an accurate measurement because there is no support on which the patient can rest his finger. As a result of the tension in the finger, inaccurate blood pressure measurements are taken.
In the conventional blood pressure finger cuff described above, the range of finger sizes to which the cuff can be applied properly is limited. If a person's finger size happens to match the finger cuff size, pressure would be applied to the finger as the cylindrical cuff or chamber is filled with compressed air. If the finger to be measured does not match the finger cuff size, but is, for example, thinner than the finger size optimal for the finger cuff, the chamber is stretched and deformed by the pressure applied by the compressed air before the cuff is expanded enough to cause its surface to press against the finger. The tension caused by the deformation of the chamber affects the pressure inside the finger cuff, and the pressure applied to the finger is reduced. If pressurization were continued while the chamber was deformed, the sum of the pressure against the finger and the pressure required to expand the cuff would equal the internal pressure of the cuff. The result would be that the internal pressure in the cuff would not correspond to the blood pressure value. Since this type of chamber deforms when it stretches, the insertion of a thin finger in the cuff causes the midline cross-section to expand like a balloon, while the two ends of the chamber, along the path of insertion, become round. Thus, both the shape of the cylindrical cuff and its length when pressed against a finger vary with the diameter of the finger. This variation leads to measurement errors.
An example of this can be seen in FIG. 17. When chamber 501 expands like a balloon, the length of the surface in contact with the finger, which was originally l.sub.1, is reduced to l.sub.2.
Another disadvantage of this type of cuff is that the material of the air containment wall or chamber is limited to materials which can stretch, yet are highly stable, i.e., materials capable of substantial elastic deformation.
As seen in FIG. 18, a conventional finger cuff has many parts to assemble, in this case, five pieces. In this conventional finger cuff, a long, flat air chamber 590 is rolled into a cylinder 591a, 591b, both ends of which are taped on both surfaces to form a permanent tube then sealed by end pieces 592, 593. The task of winding the cuff into a cylinder is extremely complicated, and thus, assembling the cuff is difficult.